Residents have called for a fresh review of military training exercises over nature reserves after an investigation found a fatal helicopter crash was caused by a bird strike.

Captains Christopher S Stover and Sean M Ruane, technical sergeant Dale E Mathews and staff sergeant Afton M Ponce all died when a US Air Force (USAF) HH-60 Pave Hawk helicopter came down on a nature reserve at Cley-next-the-Sea during a training exercise in Norfolk on January 7.

An Accident Investigation Board report into the accident today found that a flock of birds, startled by the noise of the aircraft, had struck the helicopter, breaking through the windscreen and knocking the pilot and co-pilot unconscious.

The stabilisation systems disabled by the strike, the helicopter began to move randomly and crashed three seconds later.

Shortly after the accident, Brendan Joyce, chief executive of the Norfolk Wildlife Trust which runs the Cley Marshes nature reserve, said the trust had lobbied the Ministry of Defence (MoD) and local MPs about low-flying training missions in the area a year earlier.

A trust spokesman today said it had not seen the report ahead of its publication and was still considering any response.

Richard Kelham, chairman of Cley Parish Council, said the findings confirmed residents' long-held fears over low-flying helicopters over the marsh and he hopes the matter can be looked at once more.

He said: "These findings strengthen our hand in the argument against low flying over nature reserves.

"Our concerns are for both the welfare of the wildlife but also from a safety point of view.

"It is inherently dangerous to fly low over an area with a lot of birds and hopefully lessons can be learned from this tragedy."

Brigadier General Jon Norman, president of the inquiry's board, said his investigation had "found clear and convincing evidence" that "multiple bird strikes caused the mishap".

As well as the loss of life, the crash cost the US government an estimated at 40,302,061 dollars (£23.5 million), the report said.

The crew were serving with 56th Rescue Squadron, 48th Fighter Wing, based at RAF Lakenheath in Suffolk, which carries out search and rescue operations

At the time they were on a training mission to recreate a night-time rescue scenario involving a downed F-16 pilot.

As they flew over Cley Marshes the helicopter was engulfed by a flock of geese, which weighed up to 12lbs each, after the birds were "likely startled by the noise of the approaching helicopters".

"At least three geese penetrated the windscreen, rendering the mishap pilot and mishap co-pilot unconscious, and at least one goose struck the mishap aerial gunner in the performance of special mission aviator duties, rendering the mishap aerial gunner unconscious," it adds.

With the pilot and co-pilot unconscious and stabilisation systems disabled, the helicopter's control stick would have moved randomly, the report adds.

The helicopter banked to the left and, unable to lift vertically, hit the ground within seconds.

The report states that the crew "followed the available guidance on bird hazards in the UK" and complied with the UK Military Low Flying Handbook to the "maximum extent".

Recent bird activity maps had suggested low activity in the area but a storm surge the previous month had forced several flocks of birds to relocate from the nearby Blakeney Nature Reserve.

This relocation is likely to have resulted in a larger-than-normal presence of geese.

The MoD said it had received 11 complaints about low flying in 2013, none of which were made by people living in the Cley area. These related to the noise created by aircraft and not safety risks.

A "seasonal avoidance" had been in place between April and September each year since 2012 following an application from the wildlife trust but this accident happened outside that period.

An MoD spokesman said it had no new comment in light of the report, but previously said: " We understand that military low flying can be noisy and unpopular but it is an essential part of operational training.

"The 20th century legacy of military airfields close to this location does make this a particularly busy area for aircraft and the MoD is constantly striving to ensure that such disturbance is kept to an absolute minimum and that the burden of noise pollution is as evenly distributed as possible throughout the UK Low Flying System as a whole.

"In addition the MoD takes the protection of wildlife and habitats very seriously and liaises constantly with the joint nature conservation committee."

A wildlife trust spokesman said: "We worked closely with the USAF, MoD and emergency services in the immediate aftermath of this tragic accident and co-operated fully with the subsequent air accident investigation.

"Cley Marshes is an internationally recognised nature reserve for wildlife, in particular for birds.

"Low flying aircraft cause a disturbance to wildlife and although there is a restriction on low flying enforced for the bird breeding period, there is currently no restriction through the autumn and winter period.

"Norfolk Wildlife Trust will continue to seek a year-round restriction on any low flying aircraft, to minimise the disturbance to wildlife at Cley Marshes."